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Fabrication of a New Hydrous Zr(IV) Oxide-Based Nanocomposite for Enhanced Pb(II) and Cd(II) Removal from Waters Ming Hua, Yingnan Jiang, Bian Wu, Bingcai Pan,* Xin Zhao, and Quanxing Zhang State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210023, People’s Republic of China S Supporting Information *
ABSTRACT: To overcome the technical bottleneck of fine hydrated Zr(IV) oxide particles in environmental remediation, we irreversibly impregnated nanosized hydrated Zr(IV) oxide inside a commercial cation exchange resin D-001 and obtained a new nanocomposite NZP. NZP exhibited efficient removal of lead and cadmium ions in a pH range of 2−6, where no Zr(IV) leaching was detected from NZP. As compared to D-001, NZP showed more preferable adsorption toward both toxic metals from the background Ca(II) solution at greater levels. The synthetic Pb(II) or Cd(II) solution containing other ubiquitous metal ions was employed as the feeding influent for column adsorption, and the results indicated that the treatable volume of NZP is around 3− 4 times that of D-001 before reaching the breakthrough point set according to the effluent discharge standard of China. With respect to Pb(II) removal from an acidic mining effluent, the treatable volume of NZP was 13 times higher than that of D-001. The exhausted NZP could be effectively regenerated by HNO3−Ca(NO3)2 binary solution for repeated use without any significant capacity loss. The superior performance of NZP was attributed to the Donnan membrane effect exerted by the host D001 as well as the impregnated HZO nanoparticles of specific interaction toward toxic metals, as confirmed by the comparative isothermal adsorption and X-ray photoelectron spectroscopic study. KEYWORDS: hydrated zirconium oxide, polymeric nanocomposite, heavy metals, enhanced removal, XPS study, acidic mining effluent
1. INTRODUCTION Water and wastewater contaminated by heavy metals is a worldwide environmental concern because of their adverse impact on environmental ecosystems and public health. Various technologies such as chemical precipitation,1 adsorption2 and electrochemical method,3 membrane filtration,4 and biological treatment5 have been proposed in response to heavy metal contamination. Among the available technologies, adsorption is regarded as one of the most attractive options and has been widely used in field application. Though various adsorbents have been fabricated for heavy metals such as ion exchangers, activated carbon, and numerous low-cost adsorbents, development of new adsorbent materials is still a hot topic because the available adsorbents have some inherent disadvantages from technical and economical viewpoints. For example, the polymeric ion exchangers usually trap heavy metals through nonspecific electrostatic interaction and thus display little or insufficient specific adsorption affinity toward the target metals © XXXX American Chemical Society
in the presence of co-ions (e.g., Ca(II), Mg(II), etc.) at greater levels.6 As a result, a significant decrease in the working capacity occurs, resulting in frequent regeneration and high operation cost. In the past decades, various metallic oxides including Fe(III),7 Mn(IV),8 Ce(IV),9 and Ti(IV)10 oxides have been exploited as specific adsorbents through inner-sphere complexation with the target toxic metals. Similar to other metal oxides, hydrous Zr(IV) oxide (ZrO2·nH2O, HZO) could serve as a heavy metal adsorbent because it possess a rich amount of hydroxyl groups and shows high capacity and specific adsorption toward heavy metals such as Zn(II), Hg(II), Cd(II), and Sn(II).11−14 Also, the exhausted HZO could be efficiently regenerated by acidic solution12 because, as an amphoteric Received: September 17, 2013 Accepted: October 29, 2013
A
dx.doi.org/10.1021/am404031q | ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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with ethanol in a Soxhlet apparatus to remove the residue impurities, transferred to Na+-type in NaOH solution, and then flushed with the ultrapure water until the neutral pH (6.8−7.2) was reached. Finally, the beads were vacuum dried at 343 K until a constant weight was reached. The bulky HZO particles were prepared by direct liquid precipitation and thermal treatment for comparative study.12,24 2.2. Fabrication of NZP. In brief, the preparation procedure of NZP consisted of the following three steps: (i) Ten grams of D-001 beads was soaked into a 100 mL solution containing 25 g of ethanol and 10 g of ZrOCl2. The mixture was stirred for 12 h to allow the uptake of Zr(IV) ions onto the inner surface of D-001 through a typical ion exchange process:
compound, HZO could be protonated or deprotonated at different pHs, with its pKpzc value around 2.2−2.9.14,15 Moreover, HZO is innocuous, inexpensive, and readily available, has good resistance to oxidant, and is very stable in a wide pH range (2−14),16,17 implying its great potential in decontamination of water from toxic metals. Unfortunately, as compared to Fe, Mn, and other metal oxides, less attention was paid to HZO to examine its properties and evaluate its potential in environmental application, possibly because HZO is scarcely present in natural environments such as in soils. Also, similar to other metal oxides, HZO is usually present as fine or ultrafine particles (micrometer level), and cannot be used directly in fixed bed or any other flow-through systems due to the excessive pressure drop and poor mechanical rigidity. An effective approach to improve the applicability of fine metal oxide particles in water treatment is to impregnate them inside porous materials of larger size, such as activated carbon,18 zeolite,19 cellulose,20 and some low-cost adsorbents.21 The resultant composite adsorbents combine the specific adsorption of toxic metals by metal oxides with the advantage of the hosts with the improved permeability and feasible phase separation in flow-through systems. Recently, polymeric ion exchangers have been demonstrated as preferable host alternatives for metal oxide immobilization. It is originated from satisfactory mechanical rigidity of the polymeric hosts, as well as the potential Donnan membrane effect exerted by the immobilized charged groups covalently bound to the polymeric backbones, where target metal ions would be subjected to preconcentration and enhanced permeation prior to their effective sequestration by oxide particles.22 Also, due to the crosslinking nature and nanoporous structure of the host polymer, the metal oxides could disperse as nanoparticles with larger specific surface areas and higher surface free energy, which would greatly favor heavy metal retention as compared to the bulky particles.23 The main objective of the current study is to develop a HZObased nanocomposite adsorbent and evaluate its properties for efficient removal of toxic metals from water. A commercially available cation exchanger D-001 of porous polystyrene structure was employed as the host to immobilize HZO nanoparticles, and we obtained the resultant nanocomposite NZP. The physicochemical properties of NZP beads were characterized. Lead and cadmium ions were selected as the target pollutants because of their ubiquities in waters and negative impact on the environment. The influence of solution pH, adsorption time, and co-ions on adsorption was evaluated, and cyclic adsorption and regeneration experiments were performed to evaluate the newly developed adsorbent for lead and cadmium removal from water. The underlying adsorption mechanism was explored with the aid of X-ray photoelectron spectroscopy (XPS). Furthermore, wastewater from a local mineral processing plant was sampled for column tests to validate the feasibility of NZP for practical application.
RSO3− Na + + Zr 4 + → (RSO3−)4 Zr 4 + + Na +
(1)
(ii) The Zr(IV)-preloaded D-001 beads were filtered, desiccated under ambient temperature, transferred to a binary NaOH− NaCl solution (at 10% w/v concentration), and stirred for 12 h. Zr(IV) was then precipitated as Zr(IV) hydroxides onto the inner surface of D-001:
(RSO3−)4 Zr 4 + + NaOH + NaCl → RSO3− Na + + Cl− + Zr(OH)4 ↓
(2)
(iii) The D-001 beads containing Zr(OH)4 were dried at 60 °C for 12 h, and then the nanocomposite adsorbent NZP was obtained. Δ
Zr(OH)4 → ZrO2 · nH 2O + H 2O↑
(3)
2.3. Batch Adsorption Experiments. Batch adsorption experiments were conducted by using the traditional bottle-point method. Fifty milligrams of the adsorbent samples (D-001 or NZP) were equilibrated in 250 mL flasks with 100 mL of aqueous solutions containing Pb(II) or Cd(II) at designed levels. To study the effect of pH and coexisting Ca(II) on the metal uptake, the initial concentration of Pb(II) or Cd(II) was set as 0.5 mmol/L. In the isothermal study, the range of initial concentration of Pb(II) or Cd(II) was set from 0.25 to 4.0 mmol/L. The flasks were then transferred to a model G-25 incubator shaker with thermostat (New Brunswick Scientific Co., Inc.) and shaken at 150 rpm for 24 h at a given temperature. Ca(II) was employed as a competing ion by introducing a certain amount of Ca(NO3)2 into the test solution. HNO3 and NaOH solutions were used to adjust the solution pH throughout the experiment when needed. Adsorption kinetics was performed by mixing a 1000 mL solution that contained 3.0 mmol Pb(II) or Cd(II) and 0.50 g of NZP and determined by sampling 0.5 mL aliquots at various time intervals. The effect of solution pH on the Zr leaching from NZP was examined as compared to the bulky HZO. The uptake of the target metals was determined by conducting a mass balance. Other details about batch adsorption were described in related tables and figures. 2.4. Fixed-Bed Column Sorption. Column adsorption experiments were carried out with a polyethylene column (12 mm diameter and 130 mm length) equipped with a water bath to maintain a constant temperature. The Lange-580 pumps (Baoding, China) were used to ensure a constant flow rate. The wet D001 or NZP beads of 5 mL were packed inside separate columns before operation. A synthetic feeding solution containing 20 mg/L Pb(II) or 5 mg/L Cd(II) and other co-ions (Ca(II), Mg(II), and Na(I)) was prepared. Its detailed composition is described in the related figures. The mining effluent was sampled from Nanjing Yinmao lead−zinc mining Co., Inc. and employed for column test to further validate the feasibility of NZP in wastewater treatment. The hydrodynamic conditions (i.e., a superficial liquid velocity (SLV) of 0.45 m/h and an empty bed contact time (EBCT) of 6.0 min) were identical for all the column adsorption runs. As for the regeneration test, the binary HNO3(0.10 M)−Ca(NO3)2 (5 wt %) solution was used as the regenerant, and SLV and EBCT were set as 0.045 m/h and 60 min, respectively.
2. MATERIALS AND METHODS 2.1. Materials. All chemicals of analytical grade or more pure were used in the current study without further purification. Stock solutions of Pb(II) and Cd(II) were prepared by dissolving Pb(NO3)2 and Cd(NO3)2 into ultrapure water, respectively. The macroporous polystyrene cation exchanger D-001 was kindly provided by Jiangsu NJU Environmental Technology Co., Ltd. Prior to use, the spherical D-001 beads were sieved (0.6−1.0 mm) and subjected to extraction B
dx.doi.org/10.1021/am404031q | ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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2.5. Characterization and Analysis. The specific surface area and pore size distribution of NZP was measured using N2 adsorption and desorption tests at 77 K (Micromeritics ASAP-2010C). The morphology of the encapsulated Zr(IV) oxides was observed with transmission electron microscopy (JEM-200CX) equipped with a Gatan CCD camera working at accelerating voltage of 200 kV. The mineralogy of Zr(IV) oxides was determined by powder X-ray diffraction patterns over a wide range of angles (10−80°) using a highresolution X-ray diffractometer (XTRA) with Cu Kα radiation (40 kV, 25 mA). An inductive coupled plasma (ICP) emission spectrometer (PerkinElmer Model Optima 5300DV) was used to determine the Zr concentration in solution, and the Zr amount loaded within D-001 was measured after the nancomposite sample was digested into a HNO3− HClO4 solution. The Pb(II) and Cd(II) concentration in solution was determined by a Shimadzu AA-6800 atomic absorption/emission spectrometer equipped with a graphite furnace atomizer and deuterium background correction. The Pb(II) adsorbed samples were vacuum desiccated at 323 K for 24 h and then ground into powder for further X-ray photoelectron spectroscopy (XPS) analysis (UlVAC-PHI model 5000 Versa probe). All the binding energies were referenced to the C 1s peak at 284.8 eV to compensate for the surface charging effects. The XPS results were collected as binding energy forms and fitted using a curve-fitting program (XPSpeak4.1).
3. RESULTS AND DISCUSSION 3.1. Characterization of NZP. The resulting nanocomposite adsorbent NZP was present as spherical beads of brownish-yellow color. As shown in Figure 1a, the TEM image suggests that HZO was immobilized inside D-001 as nanoparticles (10−20 nm). Nanosized metal oxides usually display larger accessible surface areas and higher surface free energy than the bulky ones, and thus are expected to offer more active adsorption sites.25 According to the wide-angle XRD patterns of NZP, D-001, and the bulky HZO particles (Figure 1b), most HZO impregnated inside D-001 is amorphous in nature. It possibly resulted from the relatively low temperature (95%. Before the next cycle adsorption, both NZP columns were rinsed with 3 BV 0.10 M NaOH solution to neutralize the protonated HZO and transfer D-001 from H-type to Na-type. Cyclic adsorption and regeneration runs of the NZP columns were performed to validate their repeatability for heavy metal removal. The overlap of the cyclic breakthrough curves illustrated in Figure 8a,b demonstrated that NZP could be employed for repeatable Pb(II) and Cd(II) removal without any significant capacity loss. We also sampled acidic mining effluent from a mining plant in Nanjing and examined the feasibility of NZP for industrial wastewater treatment. Detailed composition of the feeding wastewater was described in Figure 10a. The breakthrough F
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Figure 8. Breakthrough curves of (a) Pb(II) and (b) Cd(II) adsorption from synthetic wastewaters onto NZP and D-001 at 298 K. Superficial liquid velocity (SLV) = 0.45 m/h. Empty bed contact time (EBCT) = 6.0 min.
Figure 10. Fixed-bed column retention of Pb(II) from practical mining drainage: (a) adsorption by NZP or D-001; (b) regeneration of the exhausted NZP column.
point was set as 0.5 mg/L Pb(II), the maximum concentration level regulated by the mining industry emission standard of China.43 Results indicated that the effective treatable volume of NZP was around 2000 BV, whereas that of D-001 was only 150 BV. Also, other toxic metals including Cr(III) and Cu(II) initially at 0.026 and 0.690 mg/L, respectively, were completely eliminated by NZP adsorption. Note that for the breakthrough curves of both D-001 and NZP columns, higher Pb(II) concentration in effluent than in the feeding solution occurred after reaching the breakthrough point. Similar results were reported earlier44 and could be ascribed to the chromatographic elution effect of Pb(II) by other coexisting ions. In other words, a fraction of Pb(II) initially loaded on both adsorbents was jostled by other co-ions when insufficient adsorption sites were available. The binary HNO3 (0.1 M) + Ca(NO3)2 (5 wt %) solution was used to regenerate the exhausted NZP with the efficiency higher than 95%. The results further suggested the potential of NZP for enhanced removal of heavy metals from industrial wastewaters.
4. CONCLUSION Nanosized hydrous zirconium oxide could be encapsulated inside a polymeric cation exchanger D-001 to improve its applicability for decontamination of water from heavy metals like lead and cadmium. The resultant nanocomposite adsorbent NZP could effectively capture both toxic metals in pH 2−6 without any Zr(IV) leaching from the NZP, and it was achieved through two distinct processes: nonspecific ion exchange by the sulfonate groups of D-001 and specific inner-sphere complexation by the entrapped HZO nanoparticles. Consequently, NZP exhibited preferable adsorption to both toxic metals than other co-ions like calcium. Cyclic column adsorption tests that used the synthetic and real acidic mining effluent as the feeding
Figure 9. In situ regeneration of the exhausted columns packed with NZP: (a) Pb(II); (b) Cd(II).
G
dx.doi.org/10.1021/am404031q | ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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solution further demonstrated that NZP could be employed for repeated use without any capacity loss, when the binary HNO3−Ca(NO3)2 solution was employed as the regenerant.
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ASSOCIATED CONTENT
S Supporting Information *
N2 adsorption−desorption isotherms by NZP at 77 K; isotherm constants for Pb(II) and Cd(II) uptake onto NZP at 303 K; adsorption isotherms of (a) Pb(II) and (b) Cd(II) by NZP at 303 K. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*B. Pan. E-mail:
[email protected]. Tel: +86-25-8968-0390. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The study was financially supported by Jiangsu NSF (BK2012017), NSFC (21177059), and Changjiang Scholars Innovative Research Team in University (IRT1019)
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